The present invention relates to current sources/sinks for wireless communication systems and more particularly to such sources/sinks wherein the output impedance remains high at RF frequencies. As used hereinafter in the specification and claims, "current source" is intended to be generic to both current sources and sinks.
High impedance current sources are often desirable in RF communications systems. The use of bipolar junction transistors ("BJTs") to amplify the RF signal is well known. However, BJTs with a Ft sufficiently high for such applications suffer as a result of the reduction in gain at such high frequencies. In many applications involving integrated circuits, such high frequency BJT transistors are the only transistors available on the integrated circuit and the lack of output impedance when in use as a current source remains a problem.
BJTs are of course well known and have base, collector and emitter terminals. Structurally, the emitter and collector terminals of early BJTs were identical and interchangeable and interchanged. Since, however, the gain of a transistor decreases with frequency to unity at the transfer frequency Ft, it is generally desirable to operate transistors at a small fraction of Ft in order to achieve amplification. As a result, the emitters and collectors of modern BJTs with a Ft of at least 1 GHz, perhaps as much as at 10 GHz, are quite different structurally in integrated circuits designed for operation at frequencies of at least 300 MHz, and desirably 500-1,000 MHz. These physical differences result in a transistor in which the operating characteristics are very different when the transistor is inverted, i.e., when the collector and emitter terminals are interchanged.
It is also known to use inverted BJTs in current sources for the purpose of reducing the power supply requirements of the circuits and thus power dissipation. By way of example, the Banker et al U.S. Pat. No. 5,317,208 discloses the use of such sources for logic gates.
Accordingly, it is an object of the present invention to provide a novel current source and method in which the output impedance of high frequency, integrated circuits useful in wireless communications systems can be significantly increased.
It is another object of the present invention to provide novel RF circuits and methods of using inverted BJTs.
It is yet another object of the present invention to provide a novel structure and method for current sources with higher output impedance and with a lower transition frequency.
It is yet still another object of the present invention to provide a novel structure and method for increasing the common mode rejection of a differential amplifier at frequencies above 300 MHz.
It is still another object of the present invention to provide a novel heterojunction transistor and method with higher output impedance and a reduced transition frequency.
It is yet still another object of the present invention to provide a novel integrated circuit differential amplifier and method in which the bipolar junction transistors of a current source for biasing the normal mode bipolar junction transistors of the amplifier are operated in the inverted mode to increase the common mode rejection of the amplifier at frequencies above about 300 MHz.
It is yet still a further object of the present invention to provide a novel integrated circuit transconductance amplifier and method in which the bipolar junction transistors of a current source for biasing the normal mode bipolar junction transistors of the amplifier are operated in the inverted mode to increase the common mode rejection of the amplifier at frequencies above about 300 MHz.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.